In cellular telephones, the output power of the radio frequency ("RF") power amplifier is automatically selected from a plurality of discrete power levels. By selecting the lowest effective output power level and controlling the output power, the transmitting range of a cellular telephone may be carefully limited so that radio channels may be reused throughout the cellular system and the capacity of the system may be maximized. Since cellular telephones are used year round in a variety of different environments, it is necessary to accurately control the output power within specified tolerances over a broad range of operating temperatures.
In most cases, radiotelephones employ feedback circuits to control the output of the power amplifier. A portion of the RF output energy is applied to a detector circuit, which provides a power level signal corresponding to the output power level. The power level signal is compared to a reference signal in order to generate a control signal. The control signal is then used to vary the amplifier's output power and cause the difference between the reference signal and the power level signal to be minimized.
Most prior art transmitters that incorporate RF output power control use some form of diode detector to convert the RF carrier to a corresponding DC level. If the detector requires good sensitivity to low level RF voltages, then almost invariably the diode detector is forward biased. This forward bias generates a DC voltage drop across the diode. The DC voltage drop will vary with temperature by approximately -2.5 millivolts/degree C. Thus, over a -30.degree. C. to +60.degree. C. range, the output voltage can change 225 millivolts. Consequently, the level of the power-control signal varies as the temperature varies. This in turn causes the output of the RF amplifier to change even though the reference signal remains constant.
Several methods of compensating for these temperature induced variations are known in the art. In one method, the output from the RF amp is half-wave rectified by a first diode and a capacitor. The voltage at the anode of the first diode is biased to a predetermined voltage by means of circuitry that includes a second diode. If the first and second diodes exhibit similar temperature characteristics, the variation in voltage drop across the second diode will alter the forward bias voltage of the first diode and minimize variations in the detector output voltage level. An example of this method is described in U.S. Pat. No. 4,523,155 to Walczak et al. One drawback of this method is the relative difficulty and expense associated with selecting and matching each pair of diodes. In addition, there are practical limits on the manufacturer's ability to match the diodes and the ability to subject both diodes to the same ambient temperatures in the presence of other components.
In a second method, the output of a comparator is used to control the output power of the amplifier. A signal corresponding to the output power is rectified by a diode detector. This rectified signal is fed into one of the comparator inputs. A reference signal is fed into the other comparator input. Each of the comparator inputs has a biasing diode connecting it to ground. The biasing diodes are in close thermal contact on the same chip. This has the effect of reducing the thermal coefficient of the diode detector and the variations in the detector output voltage due to temperature. This approach is illustrated in U.S. Pat. No. 4,992,753 to Jenson et al. However, this circuit is subject to some temperature induced variations since the diode detector is not thermally compensated and does not match the biasing diodes. In addition, the diode detector is not forward biased and will exhibit non-linear characteristics at low output voltages.
In a third approach, the output of the power amplifier is sampled and is amplitude modulated at a preselected frequency. The detector circuitry produces signals whose amplitude is indicative of the RF amplifier's output power and whose frequency is the modulation frequency. The amplitude of the lower frequency waveform corresponds to the amplitude of the RF amplifier output power. An error signal, generated by comparing the lower frequency signal to a reference voltage, is used to control the RF amplifier. In this arrangement, the amplitude of the lower frequency signal is independent of the quiescent voltage drop across the detector diode. Although the amplitude is dependent upon the rectification efficiency of the diode, this effect is only slightly temperature dependent. Thus, there is very little effect from temperature changes. This method is illustrated in U.S. Pat.No. 5,003,270 to Braathen et al. This approach requires more complicated circuitry and a larger number of parts than other approaches.
Thus, there is a need in the art for an improved RF detector for power level control that effectively minimizes variations due to temperature and employs a simple circuit design that may be easily and inexpensively manufactured.
In addition, diode detector circuits do not provide any amplification of the detected signal and do not perform the comparison between the detected signal and the reference signal. Therefore, there is a need for an amplifying RF detector/comparator so as to reduce the number and/or gain of subsequent stages.